The present invention relates to monovinylidene aromatic polymers, such as polystyrene, and plastic molding compositions based thereon. An important problem encountered with such polymers involves trying to maximize toughness and resistance to heat distortion while at the same time maximizing processability, usually assessed in terms of melt flow rate. Typically, processability, i.e., melt flow rate, is increased through the use of plasticizers such as mineral oil. The problem with adding plasticizers is that they lower heat distortion resistance, flexural and tensile strength. In addition to creating usage problems for any product molded from such compositions, they also require increased mold cycle times in order to prevent post molding heat distortion. The part must be cooled down to a lower degree before being removed from the mold.
In an article entitled "The Effect of Molecular Weight Distribution on the Physical Properties of Polystyrene," Journal of Polymer Science, Volume XXXIX, pages 87-100 (1959), Herbert W. McCormick, Frank M. Brower and Leo Kin suggest that a narrowing of the molecular weight distribution should lead to higher physical properties in relation to the melt viscosity or ease of fabrication of the plastic composition. However after testing this theory, they report that:
"The lowering of the property at a given number average weight and the increase in high shear viscosity at a given weight average molecular weight with increasing narrowness of the molecular weight distribution indicate little or no merit in the idea of obtaining comparable physical properties with greater ease of fabrication for polystyrene of narrow molecular weight distribution."
The results observed by McCormick et al. have been confirmed in other literature.
It has been recognized that the anionic polymerization of styrene does enable one to control molecular weight and molecular weight distribution more closely. Very small quantities of such materials are sold for calibrating molecular weight determination instruments. However, anionic polymerization has not been used to produce polystyrene commercially. It is generally believed that the resulting polystyrene does not afford good melt flow rate for a given set of strength properties. Further, commercial scale anionic polymerization results in undesirable end product discoloration.